11        CHAPTER:  Man Made Structures

11.1    Introduction

While there is the occasional need for man made structures in creeks to assist with their rehabilitation, it is more common to find man made structures in Streams and in particular Rivers where there are more consistant flow regiemes.  As it is currently relatively uncommon for man made rehabilittion structures to be built in Creeks, with the exception of Rock shutes this topic is only covered briefly in this book and the reader is directed to “A rehabilitation manaual for Australian streams” which cover this topic in detail.

Not only have recent years seen a trend away from in stream structures in Creeks, it is particularly true tht the use of man made materials such as concrete and steel have been declining in popularity.  The reason for the decline in the use of concrete and steel is to do with a changing of focus, away from trying to force the flow with permanent structure which were often getting damaged by and often causing erosion, to more “organic structures”  which aim to work more with the flow and create long lasting systems which orgment the existing natural systems.

11.2    Soft Structures

11.2.1    Vegetation

Vegetation is often most commonly considered as an important aid to improving biodivirsity which of course it is, however vegetation is also a critical component in the structural integrity of of a Creek system.  In particular the roots of different species play a vital role in helping the banks resist different types of erosion.  With the structural importance of plant roots recognised, plants are now being used not only to bind soil together but also to bind other materials such as rock into place, essentially forming a living, reproducing structural framework.

In short the vegetation in the Creek system should be viewed as vast interconnected structural element, capleable of helping, or hindering the rehabilitation process.


11.2.2    Bank Shaping

One of the key aims of any Creek rehabilitation process is to work towards getting the Creek banks to “lie back” , away from the verticle towards about a 45 degree angle, known  as the Angle of repose.  The importance of this 45 degree angle is that around 45 degrees in most situations the bank becomes far more stable both less likely to be undercut and also to support a health cover of vegetation.  

In some situations such as where there is little existing vegetation and good access for equipement there is definitely scope to artificially shape the bank using earth moving machinary, pulling back the Lip of the bank to create a bank with an angle of approximately 45 degrees.  By using earth moving machinary to create banks with an Angle of repose can definatley speed up the rehabilitaion process however when considering the use of this process not only do the relatively high costs need to be taken into consideration but also ways in which the newly exposed earth can be quickly protected in order to resist scouring by the next high flow event.

When shaping banks it is important to revegetate the bank as quickly as possible with appropriatte species and to consider the use of sterile Rye grass or similar to obtain almost immediate protection while the native plants get established.


11.2.3    Graded Access Tracks   

Access tracks are a man made feature that can be used in Creek rehabilitation projects but which can cause often heated debate.

An Access track is a graded walking track 2-3 meters wide which runs along the Creek, often along the verge but where appropriate on severely incised site in the channel itself.  It is important to not that this Access track is not intended to be used by road registered vehicles and as such should be narrow enough to prevent their passage.

The principle advantages of an Access track are:

  • Significantly cheaper and easier weed control, both initially and over time
  • Cheaper and easier planting
  • Cheaper and easier pest contol
  • Easier and more enjoyable long term maintenance
  • Significantly higer levels of landholder recreational use, enjoyment and commitment to the project

The Principle disadvantages of an Access track are:

  • Costs associated with creating the Access track
  • Potential erosion
  • Potential disruption of animal habitat
  • May encourage other landholders to create access tracks in established and stable Creek environments or without permits

In the balance it appears that a well designed and executed access track is a valuable long term assest to most Creek rehabilitation projects assuming:

  • The Access track has been created by a recognised contractor 
  • The appropriate permits are in place
  • Attention is paid to minimizing potential erosion by carefully chosing the path of the track and ensuring that it is well seeded down with Native grasses or an appropriate sterile exotic grass

Graded Access Tracks are a relatively new concept and as yet there is no consensus amongst profesionals on their benefits or design.

11.3    Hard Structures

11.3.1    Rock chutes

Rock chutes are currently widely used to stop significant Head cut erosion.  A Rock chute uses heavy earth moving machinary to regrade the bed of the creek to a more managable gradient and to then line the bed of the creek with heavy pieces of “shatted” rock.  Shatted rock has many shape edges which enable the rocks to be wedged together helping them resist being moved in the next flood event.  By providng one or more rock chutes over a given distance, the water in the creek can be saftly transitioned over a potentiall damagingly steep gradient.

Typically the head of the Rock chute is raised slightly above, xxxx mm, proud of the upstream Creek bed, in this way an Inertial pool is created, helping to slow the water before it begins its journey down the Rock chute. 

11.3.2    Green weirs

A Green weir is a device used to help water transition across a short distance of Creek bed where a minor incidence of Step erosion has occurred or is likely to occur.  At this point on the Creek bed the gradient is such that vegetation alone is unlikely to be able to stop the bed of the Creek beginning to, or coninuing to, erode.

A Green wier differs from a Rock chute in a couple of important ways; the length of rock run along the Creek Bed is shorter, typically no longer than 10m, the change in depth of the Creek bed along the weir is less than 1m, smaller round rocks about the size of a grapefruit or smaller are used and vegetation is a key component of binding the rocks together.

The aim of a Green wier is to use vegetation reinforced with stone to create a permanent transition point at a particularly steep point in the Creek bed.  The top of the Green wier is set flush with the bed of the upsteams section of creek, in this way there is no impediment to even low flow events, and in high flow events

Green weirs are a relatively new concept and as yet there is no consensus amongst profesionals on their benefits or design.

11.3.3    Green fords

A Green ford is a rock crossing in a creek enabling an Access track to cross from one side of the creek to another.

A Green ford is created by tipping Padock rock on an existing Grassy slide or Riffle.  A Bobcat or similar is used to placce the Padock rock and then compacts the rock into the vegetation or gravel by driving the Bobcat or similar and then compacting the rock by driving the Bobcat

Green fords are a relatively new concept and as yet there is no consensus amongst profesionals on their benefits or design.